The influence of seismic wavelet on the resolution of pseudoimpedance section for construction of underground storage

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1 GOSPODARKA SUROWCAMI MINERALNYMI Tom Zeszyt 2/3 EWA KAWALEC-LATA A* The influence of seismic wavelet on the resolution of pseudoimpedance section for construction of underground storage Introduction Chemical and physical properties of salt deposits have been a subject of careful examination. Particularly favourable for construction of underground reservoirs are following properties: chemical interia, very low permeability, homogeneity of large parts of deposit. Rock mass containing salt deposits display high seismologic inhomogeneity. Due to considerable contrast of elastic properties of salt versus neighbouring layers within the Zechstein rocks high reflection coefficients are observed. Inhomogeneities in geometrical sense are related to thickness changes, dying-out of strata, presence of lenses a.s.o. Geometrical changes depend on sedimentary processes as well as on those later alterations within rocks salt deposits, which in turn lead to the origin of secondary, descendant salts. Within salt deposit inhomogeneities as well as changes of thickness and lithology should be especially examined and predicted before the underground reservoir location is fixed. A method of inversion of seismic section aiming at obtaining pseudo-acoustic impedance section gives the possibility to bind very closely the acoustic impedance changes with lithologic and facial changes in subsurface. Process effectively transforms seismic information into the geological domain. There are several ways to achieve this objective. (Veeken, * Dr in., University of Science and Technology, Krakow, Poland; ewal@geol.agh.edu.pl

2 388 Da Silva 2004). This method is commonly used for the recognition of hydrocarbons. There is the new concept to adapt it to recognition of inhomogeneities within rocks salt deposits (Kawalec-Lata³a 2006). Pseudo-acoustic impedance is a convolution of real acoustic impedance and seismic wavelet. It should be realised that the inversion procedure is not a unique process. There is no single solution to the given problem. Care should be taken conditioning is a prerequisite for quantitative interpretation of the final results. Feasibility studies with synthetic modelling are recommended before embarking on inversion. Seismic traces are converted first into pseudoreflection coefficient time series, than into acoustic impedance by the inversion of the time series. The basic concept is simple; a reversal of the procedure long used to compute synthetic seismograms from sonic logs (Becquey et al. 1979; Oldenburg et al. 1983). In practice the reverse process is difficult, since it requires inversion of a low grade, the seismic trace, which is often, distorted by noise and wave propagation phenomena, into the higher-grade sonic log signal (Berteussen, Ursin 1983; Jêdrzejowska-Zwinczak 1984). Modern seismic field data can be processed to eliminate much of the distortion, and can be compensated in part wave propagation phenomena and density, to yield an approximation to reflection coefficients. An important procedure to help repair the damage to the spectrum (caused by transmission trough the earth and the recording instruments) is deconvolution. Ideally, an acoustic impedance interface should be represented by a single spike on the seismic trace. 1. Rock salt deposit in Sieroszowice area The Sieroszowice deposits contain a set of rock salt layers belonging to the cyclothem (PZ 1, PZ 2, PZ 3, PZ 4). The oldest cyclothem PZ 1 is developed almost completely. Rock salt deposit in Sieroszowice forms set rock salt layers dipping NE with an angle of 3 6, with general strike NW-SE (Preidl 1990). The top and bottom of the rock salt is shallowest at the southern boundary of area (630 m and 625 m respectively), and the deepest position of the top and bottom of the salt occur in the northern part of the deposit (1230 m and 1100 m respectively). The thickness of rock salt varies from 0 to 186 m. An area of extremely high thickness of salt is most convenient for location of underground storage of liquid fuels. Internal structure of the salt deposit displays several lithological types of rock salt (pure coarse-crystalline salt, anhydrite salt, salt intercalated with anhydrite). Secondary salts are present at places.

3 Modelling The synthetic pseudo-impedance acoustic sections presented in this paper are obtain by inversion method based on recursive relation in equation (2). The reflection coefficient for normal incidence is defined as i 1 i 1 i i R i i 1 i 1 i i (1) where: i, i rock density and velocity of i th layer z i i acoustic impedance. Rearanging equation (1) z i 1 Rk 1 R 1 zi 1 z1 1 R 1 R k k k (2) gives the recursive relation to calculation of pseudo-impedance acoustics section from reflection coefficients series or its approximation by seismic trace. The synthetic seismic trace is given by convolution reflection coefficients series and seismic wavelet where: r(t) reflection coefficients series, w(t) seismic wavelet, x(t) synthetic seismic trace. xt () rt () wt () (3) The basic concept of the reverse process is simple but in practice difficult. It should be realised that the inversion procedure is not a unique process especially due to limited bandwidth of the seismic data to typical seismic frequencies. The quality and quantity of interpretation of the final results increase with the bandwidth and dominant frequency increase. For the calculations of synthetic pseudoacoustic impedance the computer programme INWERS was used. Previously the computer programme INWERS was elaborated in the frame project (Projekt nr 4 T12B , Kawalec-Lata³a ). The programme consist of two main parts: 1) modelling, 2) visualisation of results.

4 390 Synthetic seismogram was calculated as a convolution of reflection coefficient series with seismic signal. The signal of Puzyriev s(t) was applied 2 2 st () exp( t )sin( 2 f t ) (4) 0 0 The parameters of signal are: f 0 dominant frequency, 0 initial phase, dumping factor. The ratio f 0 / determine the lengh of signal. When the ratio is 1 the signal is relatively short, the ratio 2 gives the long signal. 3. Results modelling example In the whole area of deposition of Zechstein evaporates on LGOM areas, within rock salt beds some inter-calculations of anhydrite occur. Their presence have been taken into account while creating theoretical models Fig. 1 on the base of introducing of typical elastic parameters values for Zechstein evaporates and upper layers (Grad 1987; Krynicki 1980). A number of pseudo-acoustic impedance sections for different signal parameter were constructed in order to test the possibility of determination of anhydrites distribution in sections of pseudo-acoustic impedance. Fig. 1. Simplified seismological model from Sieroszowice area Rys. 1. Uproszczony model sejsmogeologiczny z rejonu Sieroszowic

5 391 Fig. 2. Synthetic pseudo-impedance acoustic section with simplified seismological model for one trace, reflection coefficients and seismic signal. The parameters of signal applied to construction of synthetic seismogram are: f0 dominant frequency 40 Hz, 0 initial phase 0, dumping factor 40 s Rys. 2. Syntetyczna sekcja pseudoimpedancji akustycznej uzupe³niona o wykresy uproszczonego modelu sejsmogeologicznego, sejsmogramu impulsowego oraz sygna³u sejsmicznego. Parametry sygna³u stosowanego do konstrukcji sejsmogramu syntetycznego: f0 czêstotliwoœæ dominuj¹ca 40 Hz, 0 faza pocz¹tkowa 0, wspó³czynnik t³umienia 40 s

6 392 Fig. 3. Synthetic pseudo-impedance acoustic section with simplified seismological model for one trace, reflection coefficients and seismic signal. The parameters of signal applied to construction of synthetic seismogram are: f0 dominant frequency 40 Hz, 0 - initial phase 0, dumping factor 20 s Rys. 3. Syntetyczna sekcja pseudoimpedancji akustycznej uzupe³niona o wykresy uproszczonego modelu sejsmogeologicznego, sejsmogramu impulsowego oraz sygna³u sejsmicznego. Parametry sygna³u stosowanego do konstrukcji sejsmogramu syntetycznego f0 czêstotliwoœæ dominuj¹ca 40 Hz, 0 faza pocz¹tkowa 0, wspó³czynnik t³umienia 20 s

7 393 Fig. 4. Synthetic pseudo-impedance acoustic section with simplified seismological model for one trace, reflection coefficients and seismic signal. The parameters of signal applied to construction of synthetic seismogram are: f0 dominant frequency 40 Hz, 0 - initial phase 0, dumping factor 80 s Rys. 4. Syntetyczna sekcja pseudoimpedancji akustycznej uzupe³niona o wykresy uproszczonego modelu sejsmogeologicznego, sejsmogramu impulsowego oraz sygna³u sejsmicznego. Parametry sygna³u stosowanego do konstrukcji sejsmogramu syntetycznego: f0 czêstotliwoœæ dominuj¹ca 40 Hz, 0 faza pocz¹tkowa 0, wspó³czynnik t³umienia 40 s

8 394 Fig. 5. Synthetic pseudo-impedance acoustic section with simplified seismological model for one trace, reflection coefficients and seismic signal. The parameters of signal applied to construction of synthetic seismogram are: f0 dominant frequency 40 Hz, 0 - initial phase 0, dumping factor 40 s Rys. 5. Syntetyczna sekcja pseudoimpedancji akustycznej uzupe³niona o wykresy uproszczonego modelu sejsmogeologicznego, sejsmogramu impulsowego oraz sygna³u sejsmicznego. Parametry sygna³u stosowanego do konstrukcji sejsmogramu syntetycznego: f0 czêstotliwoœæ dominuj¹ca 40 Hz, 0 faza pocz¹tkowa 0, wspó³czynnik t³umienia40 s

9 395 First seismic traces are constructed for simplified data from Sieroszowice area and basic wavelet given by equation (4). The seismic traces were converted into pseudoacoustic impedance sections Fig. 2, 3, 4, 5. The colour scale reflecting the relative velocity (or acoustic impedance) variations, simplified seismological model for one trace recalculated for time scale, reflection coefficients and seismic signal was added to presented synthetic pseudoacoustic impedance sections. The recursive algorithm, based on equation (2) was used for calculation of pseudoacoustic impedance. For signal with low dominant frequency f 0 = 40Hz two values of factor =40and = 20 were chosen for short and long signals respectively. Proper pseudoacoustic impedance sections are presented in Figs. 2 and 3. For signal with hight dominant frequency f 0 = 80Hz two values of factor =80and =40 were chosen for short and long signals respectively. Proper pseudoacoustic impedance sections are presented in Figs. 4 and 5. The analysis of above presented sections apparently shows the improvement of their quality on frequency increase. This effect can be strongly diminished by long seismic signal. Conclusions Pseudo-acoustic impedance is a convolution of real acoustic impedance and seismic wavelet. These modelled sections of synthetic pseudo-acoustic impedance for salt deposit in Sieroszowice area show how changes of thickness and lithology can be visible on such transformation. The process effectively transforms seismic information into the geological domain. It facilitates the interpretation. Despite of band limited seismic signals pseudo impedance sections serve very well to map geologic structure. To compare the synthetic pseudo-acoustic impedance section one can see the influence of the seismic wavelet parameters on the resolution. The resolution apparently increases with the improvement of seismic signal quality, especially for short wavelets. Even thaw the dominant frequency is low, but a signal is short the resolution sufficient for the interpretation. Presented works give the positive answer about the possibility of predicting the inhomogeneities in deposits of Zechstein salts. Feasibility studies with synthetic modelling are recommended before embarking on inversion. REFERENCES Becquey M., Lavergne M., Willm C., 1979 Acoustic impedance logs computed from seismic traces. Geophysics vol. 44, 9, B e r t e u s s e n K.A., U r s i n B., 1983 Approcimate computation of the acoustic impedance from seismic data. Geophysics v. 48, 10,

10 396 G r a d M., 1987 Prêdkoœæ fal sejsmicznych w pokrywie osadowej platformy wschodnioeuropejskiej. Kwart. Geol., t. 31, Jêdrzejowska-Zwinczak H., 1984 Zastosowanie metody sejsmoakustycznej w zagadnieniach prognozowania przekroju geologicznego. Prace IGNiG, nr 52, s K a w a l e c -L a t a ³ a E., 2006 Wykrywanie zmian litologicznych w pok³adach soli metodami powierzchniowymi. ZSMGiG, nr XXIX, Geotechnika i budownictwo specjalne, K a w a l e c -L a t a ³ a E., 2007 Informatyczny system modelowania sekcji pseudo-impedancji akustycznej w celu optymalizacji interpretacji w rejonach perspektywicznych dla lokalizacji podziemnych zbiorników. Projekt badawczy Nr 4 T12B K r y n i c k i T., 1980 W³asnoœci sprê yste utworów cechsztyñskich. Kwart. Geol., tom 24, nr 3, Oldenburg D.M., Shauer T., Levy S., 1983 Recovary of the acoustic impedance from reflection seismograms: Geophysics, v. 48, 10, P r e i d l M., 1990 Dokumentacja geologiczna soli kamiennej wystêpuj¹cej ponad z³o em rud miedzi kopalni Sieroszowice C1. CAG PIG, Warszawa. V e e k e n P.C.H. and Da S i l v a M., 2004 Seismic inversion methods and some of their constraints. First break, vol. 22, WP YW SYGNA U SEJSMICZNEGO NA ROZDZIELCZOŒÆ SEKCJI PSEUDOIMPEDANCJI AKUSTYCZNEJ W REJONIE BUDOWY PODZIEMNYCH MAGAZYNÓW S³owa kluczowe Podziemne magazyny, sekcje pseudoinpedancji akustycznej, sygna³ sejsmiczny Streszczenie Rozpoznanie niejednorodnoœci budowy pok³adowych z³ó nabra³o szczególnego znaczenia z chwil¹ pojawienia siê koncepcji budowy podziemnych zbiorników. Podziemne zbiorniki s¹ rozpatrywane jako sk³adowiska odpadów oraz magazyny do przechowywania rezerw wêglowodorów. Z³o a soli kamiennej spe³niaj¹ odpowiednie warunki do budowy takich magazynów. Cechsztyñskie z³o a soli kamiennej na monoklinie przedsudeckiej s¹ rozpoznane w granicach LGOM (Legnicko-G³ogowski Okrêg Miedziowy). Jednak e bardziej szczegó³owe dane o wewnêtrznej budowie pok³adu s¹ po ¹dane przed podjêciem decyzji o lokalizacji podziemnego zbiornika. Powierzchniowe pomiary sejsmiczne s¹ bardzo ekonomicznym Ÿród³em pozyskiwania takich informacji. Dane z powierzchniowych pomiarów sejsmicznych i sejsmicznych profilowañ otworów wiertniczych zawieraj¹ podobne informacje, ale wyra one w inny sposób. Trasa sejsmiczna jest miar¹ amplitudy refleksów na granicach warstw o ró nych wartoœciach prêdkoœci (impedancji akustycznej ),podczas gdy profilowanie akustyczne otworów wiertniczych dostarcza wprost informacji o wartoœci prêdkoœci. Inwersja sekcji sejsmicznych w kierunku otrzymania sekcji pseudoimpedancji akustycznej pozwala na wydobycie z sekcji sejsmicznych informacji bezpoœrednio powi¹zanych z geologi¹. To u³atwia interpretacjê, ale parametry sygna³u sejsmicznego wyraÿnie wp³ywaj¹ na rozdzielczoœæ. Powszechnie metoda ta stosowana jest do rozpoznawania z³ó wêglowodorów. Now¹ koncepcj¹ jest jej zastosowanie do rozpoznawania niejednorodnoœci budowy pok³adowych z³ó soli. Istnieje wiele metod wykonywania inwersji, prowadz¹cych do tego celu. Syntetyczne sekcje pseudo pseudoimpedancji akustycznej presentowane w pracy s¹ skonstruowane dla sygna³ów o ró nych parametrach dla uproszczonego sejsmogeologicznego modelu oœrodka z rejonu LGOM.

11 397 THE INFLUENCE OF SEISMIC WAVELET ON THE RESOLUTION OF PSEUDOIMPEDANCE SECTION FOR CONSTRUCTION OF UNDERGROUND STORAGE Key words Undergrounde storage, pseudoimpedance sections, wavelets Abstract The recognition of inhomogeneities within salt deposit became important especially when the idea of construction underground reservoirs has appeared. Underground reservoirs are very interesting as the objects for storage of wastes and hydrocarbons. In salt deposits there are favourable conditions for construction of such reservoirs. Zechstein rocks salts of the Sudetic Monocline have been recognised within the limits of the Legnica-Glogow Copper District (LGOM). More detailed information is needed before the underground storage location is fixed. Seismic survey is the most economical source of detailed subsurface information. Both seismic and sonic/density log data contains similar information expressed in a different form. Seismic trace measures reflection amplitude, which varies as the relation of velocity contrast across rock boundaries, whereas a sonic log measures rock velocity directly. Inversion is one of the means to extract additional information from seismic data. The process effectively transforms seismic information into the geological domain. It facilitates the interpretation, but the seismic wavelet parameters briefly influence on the resolution. This method is commonly used for the recognition of hydrocarbons. There are several ways to achieve this objective. There is the new concept to adapt it to recognition of inhomogeneities within rocks salt deposits The synthetic pseudo-impedance acoustics section presented in this paper are constructed for different seismic parameters for the simplified data from LGOM areas.